Corrosion causes billions of dollars worth of damage each year. As a result of this chemical-physical process, pipeline sections often have to be taken out of service and replaced.

Corrosion can be caused by ineffective pipeline coatings, soil conditions and the circumstances under which a pipeline’s coating is rehabilitated onsite. In practice, surface preparation and the application circumstances appear to be critical in creating a long-lasting, high-performance coating.

The chemistry of corrosion

Corrosion can also be described as oxidation because the process involves the formation of bonds between steel and oxygen. Oxygen, however, is not solely responsible for the oxidation process. In dry environments, many materials, including steel, do not rust. The cause of oxidation is to be found in the presence of water.

Coating types

In order to prevent corrosion, the bare substrate can be protected by means of several available coatings including:

Factory coatings

• Fusion-bonded epoxy (FBE), high-density polyethylene (HDPE) and urethanes.

• Spray coatings like epoxy, urethane and zinc;
• Residues of refinery like waxes and petrolatum;
• Bitumen-based coatings; and,
• Single or multiple-layer PE/butyl tapes.

The selection of the coating depends on various factors such as:

• The estimated lifetime of substrate;
• Environment;
• Material, shape, and position of the substrate; and,
• Application and repair costs.

Phenomena and problems facing pipe coating

There are a number of phenomena that contribute to the corrosion process and that should be considered when discussing the application of pipeline coatings and the corrosion process.

Salts and osmosis

The presence of salt plays an important role in a corrosion mechanism because salt particles are present in most situations and are difficult to remove. Rinsing a blasted pipeline coating with clean water will not remove the salt particles and other contaminations in the voids of the pipe. As many pipeline coatings are not 100 per cent water resistant, the presence of salt is always a risk because it attracts water. When water and salt are present, the phenomenon of osmosis occurs. Water permeability

If the slightest permeability for water exists, corrosion will occur. Regardless of how well a coating has been applied in the factory, practice has revealed that disbondment due to the presence of water may still occur. While corrosion is always caused by a combination of factors, permeability for water should be viewed as a serious hazard to pipeline health. Adhesion problems

Any pipeline coating must have good adhesion to the substrate. To obtain an effective adhesion is not easy because application circumstances must be taken into consideration, and many coatings require a perfect surface preparation. The difference between tensions of the surface and coating material also play an important factor in adhesion failures.

Surface preparation

Studies have shown that bad surface preparation appears to be a main cause of corrosion problems. Very often field-applied coatings need a well-prepared surface in order to secure excellent adhesion and sandblasting is often required. However, remaining pollutions in the voids of the blasted surface and salt particles can create problems, and rapid disbondment may occur. Microbiologically-influenced corrosion

Microbiologically-influenced corrosion (MIC) is a phenomenon in which corrosion is initiated or accelerated by the activities of micro-organisms. The first case of MIC was discovered in 1934 in which sulphate-reducing bacteria were responsible for the corrosion failure of cast iron pipe.

MIC is responsible for a large portion of corrosion problems experienced in the pipeline industry. During the metabolic process, sulphate is reduced to sulphide, which reacts with hydrogen to produce hydrogen sulphide. Hydrogen sulphide is very corrosive to ferrous metals and further reacts with dissolved iron to form an iron sulphide film over the metal pipe.

Overcoming these problems In order to prevent corrosion, steel parts must be protected from contact with water. This can be achieved through the application of a protective coating. This coating must be 100 per cent water-repellent, with the capability to perfectly-match the surface of the pipeline. It should have a perfect adhesion and reduce the risk for MIC.

VISCOTAQ presents a solution

VISCOTAQ has developed viscous elastic coatings to prevent corrosion of under and above-ground pipelines.

Designed in co-operation with European Polymer Laboratories, VISCOTAQ’s new coatings consist of amorphous a-polar polyolefins. The material has been developed in such a way that it demonstrates both elastic and fluid properties. As a solid, VISCOTAQ has a true yield point and – under pressure – cold flow characteristics, which allow it to flow into the pores and anomalies of the substrate.

Eliminating the risk of corrosion

The presence of water vapour, salt particles and the resulting process of osmosis will no longer be hazardous to pipelines because VISCOTAQ’s coatings are watertight and water-repellant. As a result, rinsing with clean water in remote areas will no longer be necessary to remove salt particles and other contaminants.

VISCOTAQ has exceptional wetting characteristics, which will enable adhesion to occur quickly and last for decades. The formula does not contain any reactive groups and will not deteriorate over time. It has been designed to adhere to the surface of a pipeline so that when peeled off, the material will break apart, leaving behind a protective film.

Due to the use of a 100 per cent inert formulation, the material will retain its plasticity and not become brittle. VISCOTAQ is protected by an outer wrap for mechanical protection. As a result, the main function of corrosion prevention is separated from the mechanical protection. Many other coatings attempt to combine both of these purposes in one coating, often resulting in the loss of one or both functions.

VISCOTAQ has developed coatings that are able to protect pipelines from MIC. The permanent wetting coat wrap comprises an organic polymeric composition with inorganic filler material. In essence, without water, bacteria cannot survive.

Ensuring perfect adhesion

The substrate should be clean to a ST-2 level. No sandblasting is required as removal of sand, loose parts and grease is sufficient. The coating should be applied above the dew point, and because VISCOTAQ has a low glass transition temperature, it can be applied within a wide temperature range. It can be wrapped at temperatures below 0 degrees Celsius but also on pipes having a temperature of 70 degrees Celsius. Due to the cold flow characteristics and the low surface tension, the coating will adhere to all materials, including polyethylene and polypropylene.

VISCOTAQ is available as a wrap coating (Viscowrap), as paste (Viscopaste), as a solid sealant for roofs and tanks (EZ-WRAP), and as a fluid sealant for cable conduits and tanks (ViscoSealant). As a mastic, it can be used to protect tank bottoms, flanges, valves and irregular spaces. As a coat wrap it can be used for the protection of pipelines and large valves.